The present invention relates to an electric brake assembly including an electric motor having a rotary member, and a mechanism for converting the rotary motion of the rotary member of the motor to a linear motion of a linearly movable member to press a first brake member such as brake pads against a second brake member such as a brake rotor.
Hydraulic brake systems account for by far the majority of the current vehicle brake systems. But with the recent increasing popularity of sophisticated high-tech brake systems such as anti-lock brake systems (ABS), electric brake assemblies are gathering attention because they need no hydraulic circuits, which require complicated control, and thus are compact. An electric brake assembly includes an electric motor having a rotary member, and a driving force conversion mechanism for converting the rotary motion of the rotary member of the motor to a linear motion of a linearly movable member to press a first brake member against a second brake member.
Since such an electric brake assembly is mounted under each spring of a vehicle suspension, it has to be as small in size and lightweight as possible. For this purpose, the linearly movable member is typically in threaded engagement with the rotary member of the motor. In many cases, as disclosed in JP patent publications 2002-213505 (Publication 1) and 2000-283195 (Publication 2), the linearly movable member and the rotary member of the motor constitute a ball screw because a ball screw can convert rotary motion to linear motion with minimum frictional resistance. Some other conventional electric brake assemblies include a driving force conversion mechanism in the form of a cam assembly which comprises two cam members having axially opposing cam faces, and a plurality of rolling elements disposed between and held in engagement with the cam faces. It is desired that such an electric brake assembly have additionally a parking brake function so that this function serves as a backup of the electric brake function in case of failure of the latter (see JP utility model publication 2546348; Publication 3).
The first brake member (which is typically a brake pad or pads) gradually becomes worn by frictional contact with the second brake member (brake rotor). As the first brake member is worn, the position of the linearly movable member when the brake is not applied (this position is herein referred to as “standby” position) has to be gradually moved toward the brake rotor to ensure sufficiently quick response of braking.
Publication 1, however, does not disclose moving the standby position toward the brake rotor according to the degree of wear of the first brake member. In this publication 1, to apply the brake, the rotor of the electric motor, which is a nut member of a ball screw, is rotated in the forward rotational direction to move the threaded shaft of the ball screw, or the linearly movable member in the forward direction. To release the brake, the rotor of the motor is rotated in the reverse rotational direction to separate the first brake member from the second brake member and then rotated in the forward rotational direction until the linearly movable member returns to the original standby position. Thus, the original standby position remains unchanged even when the first brake member becomes worn.
The electric brake assembly disclosed in Publication 2 includes an adjusting screw between the driving force conversion mechanism and a piston as the linearly movable member, and a limiter and a one-way clutch disposed between the adjusting screw and the rotor of the electric motor. The limiter transmits only a rotation of the rotor that exceeds a predetermined range. Thus, when the rotation of the rotor during braking exceeds the predetermined range, the reverse rotation of the rotor thereafter is transmitted to the adjusting screw through the one-way clutch. The piston is thus advanced toward the second brake member or disc rotor by an amount corresponding to the degree of wear of the first brake member or brake pads.
In the electric brake assembly disclosed in Publication 1, in order to move the standby position of the linearly movable member according to the degree of wear of the first brake member, it will be necessary to provide a sensor for detecting the degree of wear of the first brake member, and a controller for changing the standby position according to the degree of wear of the first brake member as detected by the sensor. The addition of the sensor and the controller would complicate the entire assembly.
The electric brake assembly of Publication 2 needs no such sensor or controller to adjust the standby position according to the degree of wear of the brake pad. But still, it needs a large number of parts and is complicated in structure. It is therefore difficult to reduce the size and weight of such a brake assembly to a sufficient degree as an electric brake assembly. The manufacturing cost also tends to be high.
Further, in either of the brake assemblies disclosed in Publications 1 and 2, the electric motor has to be turned in the reverse rotational direction to release the brake, so that the brake cannot always be released sufficiently quickly, which will give uncomfortable feeling to the driver. Also, since these brake assemblies have no parking brake functions, nothing can serve as a backup of the electric brake function in case of failure of the electric brake function.
An object of the present invention is to provide an electric brake assembly which includes the function of moving the linearly movable member according to the degree of wear of the brake pads and the parking brake function, and is still sufficiently compact as an electric brake assembly.